Recent years have seen a remarkable development of facsimile equipment and printers. In particular, these apparatus generally employ a heat-sensitive recording system using a combination of a thermal head and a heat-sensitive recording paper comprising a colorless dye such as Crystal Violet Lactone and a phenol compound coated thereon, as described, e.g., in Japanese Patent Publication No. 14039/70.
This heat-sensitive recording system is advantageous in that the recording paper undergoes a primary color formation which requires no development, simplifies the recording apparatus, and reduces the cost of the recording paper and the recording apparatus, and that it use a quiet non-impact process. Therefore, it has held a high rank among low-speed recording methods. However, a serious drawback of heat-sensitive recording lies in the slowness of the recording speed as compared with electrostatic recording and other recording methods. Also, heat-sensitive recording has not yet extended its applicable range to include high-speed recording.
A problem in such high speed heat-sensitive recording is that sufficient heat conduction cannot be made between the thermal head and the heat-sensitive recording paper with which the thermal head is in contact, making it impossible to provide sufficient recording density. This has been a major factor that prevents the heat-sensitive recording paper from being used in high speed recording. A thermal head is an assembly of dot-shaped electrical resistance heating elements which respond to a recording signal to produce heat. When these electrical resistance heating elements are brought into contact with the heat sensitive color forming layer, the heat-sensitive color forming layer is molten to develop color. In order to provide sharp and high density recording, an excellent dot reproducibility is required. In other words, the thermal head and the heat-sensitive color forming layer need to be in close contact with each other to provide a more efficient heat conduction therebetween so that dots having a shape corresponding to that of the dot-shaped heating elements can be formed on the heat-sensitive color forming layer in fully developed colors in full response to high speed recording signals. At present, however, only a small percentage of heat produced by the thermal head is transmitted to the heat-sensitive color forming layer. Thus, the efficiency of heat conductance between the two components is still extremely low.
Heretofore, several approaches have been proposed to improve the surface smoothness of the heat-sensitive color forming layer so that the thermal head and the heat-sensitive color forming layer come into as close contact as possible with each other.
For example, Japanese Patent Publication No. 20142/77 describes a method which comprises treatment of the surface of a heat-sensitive color forming layer to a Bekk smoothness of 200 to 1,000 seconds. Japanese Patent Application (OPI) No. 115255/79 describes that when a Bekk smoothness is from 200 to 1,000 seconds, the heat-sensitive color forming layer can respond only to heat impulses of from about 5 to 6 milliseconds, and that for high-speed recording using heat impulses of not more than 1 millisecond it is necessary for the surface of the heat-sensitive color forming layer to be made smooth to such an extent that the Bekk smoothness is not less than 1,100 seconds. However, when the Bekk smoothness is increased to not less than 1,100 seconds, color fog is produced upon application of pressure. The formation of color fog is prevented by using a base paper which has been previously made smooth to an extent that the Bekk smoothness is 500 seconds or more. Japanese Patent Application (OPI) No. 156086/78 describes that the surface roughness, Ra, of the heat sensitive color-forming layer is made to be less than 1.2 .mu.m, and the glossiness less than 25%.
In all the above-described prior art techniques, the smoothness of the heat-sensitive color forming layer is increased only by calender processings such as super calendering, machine calendering, and gloss calendering. This calendering is applied to the base paper alone, or the base paper and the heat-sensitive paper, or the heat-sensitive paper alone. In the heat-sensitive recording paper, however, as the smoothness is increased by the calendering in order to increase the recording density, adherence and accumulation are increased. In practical use, therefore, the smoothness is suppressed to a suitable level so that the recording density and the occurrence of adherence and accumulation are properly balanced. In the prior art techniques, regardless of the smoothness level, the resulting heat-sensitive recording paper is unsuitable for practical use for high-speed recording in respect of recording density and recording stability.
The term "adherence" (sticking) as used herein refers to a phenomenon wherein the thermal head adheres to the heat-sensitive color-forming layer, thereby producing stripping noise and lowering the dot reproducibility. The term "accumulation" (piling) refers to a phenomenon wherein heat-melted products of the heat sensitive color-forming layer accumulate on the thermal head, thereby lowering the recording density and dot reproducibility. Both of these phenomena inhibit stable recording.
Another disadvantage as encountered in applying the calender processing to the heat-sensitive recording paper is that color fog is formed by pressure, resulting in an increase in the density of the background of the recording paper. Similarly, in the calendering processing of the base paper, so-called cockle, wrinkles, etc. due to unevenness in basis weight develop. Thus it is limited in its practical use. As described above, the attempt to increase the smoothness of the heat-sensitive color-forming layer by calendering processing so as to increase the recording density has met with only limited success, and the resulting heat-sensitive recording paper is not sufficiently satisfactory for use in high-speed recording.